WO2011142855A2

WO2011142855A2 - Room temperature ionic liquids and ionic liquid epoxy adducts as initiators for epoxy systems

Info

Publication number: WO2011142855A2
Application number: PCT/US2011/023739
Authority: WO
Inventors: Guiseppe R. Palmese; M. Aflal M. Rahmathullah; Amutha Jeyarajasingam
Original assignee: Drexel University
Priority date: 2010-02-04
Filing date: 2011-02-04
Publication date: 2011-11-17
Also published as: WO2011142855A3

Abstract

Certain room temperature ionic liquids (RTILs) are utilized to initiate cure or as additives to manage mechanical properties in thermosetting epoxy resins. Curing thermosetting epoxy resins in the presence of room temperature ionic liquids which may initiate curing and/or act as additives to manage mechanical properties of the thermosetting epoxy resins is disclosed, as well as ionic liquids useful for curing thermosetting epoxy resins and/or to act as additives to manage mechanical properties of the thermosetting epoxy resins. The ionic liquids may form part of a curing accelerator. Resin compositions comprising a compound which contains an epoxy group and at least one ionic liquid are also disclosed, as are cured products obtained by curing a resin composition in accordance with the present invention. Tunable properties of the epoxy resins of the present invention include ability to vary cross-linker ratios, the ability to vary non-reactive RTIL concentration possessing desired characteristics.

Description

ROOM TEMPERATURE IONIC LIQUIDS AND IONIC LIQUI D EPOXY ADDUCTS AS INITIATORS FOR EPOXY SYSTEMS

BACKG ROUND OF TH E INVENTION

1. Statement of Government Interest

This invention was made with government support under Grant W91 1 NF-06-2-001 3 awarded by the Army Research Laboratory (ARL). The Government has certain rights in this invention.

2. Field of the Invention

The invention relates to use of certain room temperature ionic liquids and adducts of ionic liquids and epoxy resins to initiate cure in epoxy containing resins as well as to modify the properties of epoxy containing resins.

3. Brief Description of the State of the Art

Epoxy resin chemistry has been widely used in applications such as advanced composites, protective coatings, and adhesives which make use of their infusibility, solvent and crack resistance when cured. The formation of such a cured network structure is achieved using a variety of curing agents which have been the focus of a substantial amount of research. Ellis, B., ed., Chemistry and Technology of Epoxy Resins, 1 93, London; New York: Blackie Academic & Professional. An area of interest is the synthesis of one-pot formulations that are thermally latent at room temperature but react at elevated temperatures, exhibiting long term storage stability. Examples of possible initiators include

organophosphorus compounds (Yie-Shun Chin, Y.-L.L., Wen-Lung Wei, Wen-Yu Chen, "Using diethylphosphites as thermally latent curing agents for epoxy compound," Journal of Polymer Science Pari A: Polymer Chemistry, 2003. 41 (3): p. 432-440), metal complexes (Zhuqing Zhang, CP. W., "Study on the catalytic behavior of metal acetylacetonates for epoxy curing reactions," Journal of Applied Polymer Science, 2002. 86(7): p. 1 572- 1 579), imidazoles (Heise, M.S. and G.C. Martin, "Curing mechanism and thermal properties of epoxy-imidazole systems," Macromolecules, 1989. 22( 1 ): p. 99- 104), or dicyandiamide Amdouni, N., et al., "Epoxy networks based on dicyandiamide: effect of the cure cycle on viscoelastic and mechanical properties," Polymer, 1990. 31 (7): p. 1245-1253 based formulations, all of which have advantages and drawbacks. Latent curing agents are designed to remain inactive at ambient conditions and to undergo controlled reaction upon exposure to external stimuli, such as elevated temperature or radiation that leads to cure of a resin. The desired latency provides the ability to more practically and easily store and handle the materials. Latent curatives are have been developed using a variety of hardeners in either multi-component or one-pot formulations. Some examples of thermally initiated curing agents are amines, imidazoles, dicyandiamides, anhydrides, phenolic derivatives and metal complexes. One-pot epoxy formulations using imidazole or dicyandiamide initiators typically exhibit complex initiations and propagation reaction schemes.

Dicyandiamide is thermally latent because it is a solid at room temperature but melts and dissolves into the epoxy resin at elevated temperatures to initiate polymerization. This need for a phase transformation of the dicyandiamide to initiate polymerization results in practical drawbacks during epoxy resin manufacture and processing, such as difficulty dispersing the curing agent and inhomogeneous cure of the epoxy resin. A miscible liquid initiator with a similar range of latent properties could be used to solve these problems and provide significantly improved epoxy resin curing systems.

Certain room temperature ionic liquids (RT1LS) are capable of initiating the homopolymerization of epoxy resins resulting in highly cross-linked thermosets which is in some ways similar to another recent report, Krzysztof owalczyk, T.S., "Ionic liquids as convenient latent hardeners of epoxy resia" Polimery, 2003. 48( 1 1 - 12): p. 833-835.

However, the degree to which reaction occurs, the mechanism of reaction and resulting mechanical properties remain unclear, and initiation might be through a mechanism similar to BF3-amine complex formation.

U.S. Patent application publication no. US 2009/0030158 A 1 discloses resin compositions which may contain epoxy groups formulated with various ionic liquids which can be used to cure the resins to form cured products. The cured products may be used as adhesives, sealing agents and casting materials. This publication does not discuss the tunability of material properties using either different ionic liquids or molar concentrations of such ionic liquids. Rather, it focuses on the initiation and degree of cure for different types of resins. The patent also does not teach the use of monoepoxide groups or ionic liquid-epoxy resin adducts as reagents of interest.

The uses of ionic liquids (ILs) have been reviewed in Winterton, N., "Solubilization of polymers by ionic liquid, " Journal of Materials Chemistry, 2006. 16: p. 4281 -4293 and are advantageous because of their low or almost-negligible vapor pressure, extremely low viscosities, and highly tunable "designer" structures while potentially being an

environmentally benign chemical, Jonathan G. Huddleston, et al, "Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation," Green Chemistry, 2001 , 3 : p. 156-164, which means that problems associated with initiator dispersion, toxicity and possibly even cost are substantially mitigated.

SUMMARY OF THE INVENTION

Certain room temperature ionic liquids (RTILs) and adducts of ionic liquids with epoxy resins are utilized to initiate cure or act as additives to manage mechanical properties in conventional thermosetting epoxy resins.

Thus, in a first aspect, the present invention relates to a method of curing thermosetting epoxy resins in the presence of room temperature ionic liquids and/or adducts of ionic liquids and epoxy resins which may initiate curing and/or act as additives to manage mechanical properties of the thermosetting epoxy resins.

In a second aspect, the present invention relates to ionic liquids and/or adducts of ionic liquids and epoxy resins useful for curing thermosetting epoxy resins and/or to act as additives to manage mechanical properties of the thermosetting epoxy resins.

In a third aspect, the present invention relates to a curing accelerator comprising an ionic liquid and/or an adduct of an ionic liquid and an epoxy resin in accordance with the present invention.

In a fourth aspect, the present invention relates to a resin composition comprising a compound which contains an epoxy group and at least one ionic liquid.

In a fifth aspect, the present invention relates to a cured product, which is obtained by curing a resin composition in accordance with the present invention.

Tunable properties of the epoxy resins manufactured by methods in accordance with the present invention include the ability to vary cross-linker ratios, the ability to vary the sulfonic acid concentration and the RTIL concentration. The result of this is the ability to produce epoxy resins with tunable thermal, mechanical and conductive properties.

BRIEF DESCRI PTION OF TH E DRAWINGS

Figure 1 shows a DSC curve of Epon™ 828 cured with various amounts of bmimdcn in accordance with the procedure of Example 1. Figure 2 shows a DSC curve of Epon™ 828 cured with 1 5 mol% of various ionic liquids in accordance with the procedure of Example 1.

Figure 3 shows a DSC curve of Epon™ 828 cured with 1 -ethyl- 1 - methylpyrrolidinium dicyanamide (empidcn) in accordance with the procedure of Example 1 .

Figure 4 shows a DSC curve of Epon™ 828 cured with hmimCl in accordance with the procedure of Example 1.

Figure 5 shows a DMA plot of Epon™ 828 cured with 1 5 mol% of various ionic liquids in accordance with the procedure of Example 1 .

Figure 6 shows a DMA plot of Epon™ 828 cured with 1 -ethyl- 1 -methylpyrrolidinium dicyanamide (empidcn) in accordance with the procedure of Example 1.

Figure 7 shows a DSC curve of Epon™ 828 cured with an adduct of PGE and emimdcn in a 1 : 1 molar ratio in accordance with the procedure of Example 1 .

Figure 8 is a plot showing Tg (filled markers) and storage modulus at 50°C (open markers) for epoxy-amine thermosets synthesized in the presence of (circles)

(emim)ethylsulfate and (squares) (emim)tosylate.

Figure 9 shows a plot of the heat of reaction for excess ionic liquid relative to epoxy compound.

Figure 10 shows isothermal conversion profiles obtained by differential scanning calorimetry for a reaction of 1 .4 mol of emimdcn per mole of PGE epoxy resin.

Figure 1 1 shows the results of size exclusion chromatography for PGE epoxy resin cured with excess emimdcn demonstrating adduct formation.

Figure 1 2 shows a graph of the viscosity, measured using concentric cylinder geometry, at a I ^s"' shear rate at 25°C.

Figure 13 shows results of DSC temperature ramp experiments using ionic liquids having different cations with Epon™ 828.

Figures 14A- 14B show a 40 wt% IL mixture of Example 6 folded over and returning to its original shape.

Figures 1 5A-1 5B show a 45 wt% IL mixture of Example 7 folded over and returning to its original shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For illustrative purposes, the principles of the present invention are described by referencing various exemplary embodiments thereof. Although certain embodiments of the invention are specifically described herein, one of ordinary skill in the art will readily recognize that the same principles are equally applicable to, and can be employed in other apparatuses and methods. Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of any particular embodiment shown. The terminology used herein is for the purpose of description and not of limitation. Further, although certain methods are described with reference to certain steps that are presented herein in certain order, in many instances, these steps may be performed in any order as may be appreciated by one skilled in the art, and the methods are not limited to the particular arrangement of steps disclosed herein.

It must be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural references unless the context clearly dictates otherwise. As well, the terms "a" (or "an"), "one or more" and "at least one" can be used interchangeably herein. It is also to be noted that the terms "comprising", "including", and "having" can be used interchangeably.

The term "resin" as used herein means a polymer precursor compound capable of giving a three-dimensional network structure in the presence of a suitable reagent, and for example, it includes epoxy resin. In this description, the polymer precursor compound and a composition comprising it are referred to as "resin" and "resin composition", respectively: and their polymerized and cured products are referred to as "cured products".

The term "ionic liquid" (IL) generally refers to a salt comprising an anion and a cation As used herein, the term, "ionic liquid" refers to a salt comprising an anion and a cation and capable of melting at a temperature falling within a range not higher than the curing temperature of the resin. Preferably, the ionic liquid is a molten salt at an ambient temperature comprising an anion and a cation. Room temperature ionic liquids (RTILs) are ionic liquids which have melting points below room temperature, i.e. (< 23°C). RTILs are characterized by being non- volatile, they typically have negligible vapor pressure, are typically non-flammable and have a high thermal stability. In addition, RTILs may exhibit a wide temperature range for liquid phase of up to about 300°C. RTILs are highly solvating, yet non-coordinating and make good solvents for many organic and inorganic materials. RTILs also have an adjustable pH. As a result of one or more of these characteristic properties, RTILs may be used as modifiers for cross-linked polymers, as well as latent curing agent for epoxies.

Where the operating temperature is greater than the glass transition temperature of the epoxy resin, epoxy resins modified in accordance with the present invention can be used as, for example, separation membranes, fuel cell membranes, tissue surrogates and actuators. Where the operating temperature is below the glass transition temperature of the epoxy resin, epoxy resins modified in accordance with the present invention can be used in, for example, toughened systems, fortified systems, internal lubrication applications and self-healing applications.

In the composition of the invention, it is desirable that the ionic liquid uniformly dissolves in the epoxy resin at or below the curing temperature, and from the viewpoint of readily preparing the composition, the melting point of the ionic liquid is preferably lower than an ambient temperature.

The cation to constitute the ionic liquid includes ammonium cations such as an imidazolium ion, a piperidinium ion, a pyrrolidinium ion, a pyrazolium ion, a guanidinium ion, a pyridinium ion; phosphonium cations such as a tetrabutylphosphonium ion, a tributylhexylphosphonium ion; and sulfonium cations such as a triethylsulfonium ion.

Preferred imidazolium cations are imidazolium cations of the formula (I):

wherein R_\ is selected from the group consisting of methyl,' C3-C 10 linear or branched alkyl groups, cycloalkyi groups having up to 10 carbon atoms, and CpCio alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate. epoxy, ether and carboxyl groups, and R2 is selected from the group consisting of C 1 -C 10 linear or branched alkyl groups, cycloalkyi groups having up to 10 carbon atoms, and Ci-C io alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups. Ri may be selected from cycloalkyi groups having up to 10 carbon atoms, and C|-Cio alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups. When Ri is a linear or branched alkyl group, Ri may be selected from C4-C10 linear or branched alkyl groups, C6-C10 linear or branched alkyl groups or Cs-Cio linear or branched alkyl groups.

Suitable exemplary cations may include, for example, l -butyl-3-methyl-imadazolium, 1 -hexyl-3-methyl-imidazolium, 1 -(2-hydroxyethyl)-3-methylimidazolium, 1 -(3- cyanopropyl)-3-methylimidazolium, 1 -ethy 1-3 -methyl pyrrolidinium, l -butyl-3- methylpyrrolidinium and cyclohexyltrimethylammonium. The anion to constitute the ionic liquid includes, for example, alkyl sulfate anions such as ethyl sulfate, tosylate anions, tetrafluoroborate ion, dicyanamide anions, bis(trifluoromethylsulfonyl) imide anions, and halide anions such as a fluoride ion, a chloride ion, a bromide ion and an iodide ion. The preferred anions are dicyanamide and chloride anions.

Ionic liquids may be prepared in any suitable, conventional manner. For producing the ionic liquid, an anion exchange method that comprises reacting a precursor comprising a cation moiety such as an alkylimidazolium, alkylpyridinium, alkylammonium or alkylsulfonium ion and a halogen-containing anion moiety, with NaBF4, or the like, may be employed. Alternatively, an acid ester method comprising reacting an amine substance with an acid ester to introduce an alkyl group can be employed. Another method involves neutralization of an amine with an organic acid to give a salt. Other suitable, conventional methods may also be employed. In the neutralization method with an anion, a cation and a solvent, the anion and the cation are used both in the equivalent amount, and the solvent in the obtained reaction liquid is evaporated away, and the residue may be used directly as it is; or an organic solvent (e.g., methanol, toluene, ethyl acetate, acetone) may be further added thereto and the resulting liquid may be concentrated.

The ionic liquids may optionally be substituted with one or more groups selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups to provide additional reactivity and/or functionality to the ionic liquids. Ionic liquids substituted with mixtures of such groups may also be employed.

Although not specifically defined, the amount of the ionic liquid to be added to the epoxy resins may be any amount which is enough for resin curing.

The ratio of IL to epoxy monomer determines the properties of the of the resultant reaction product At high IL to epoxy molar ratios, a mixture of unreacted IL and low molecular weight material, potentially an adduct of the IL with the epoxy, is formed after sufficient heating. An example of adduct formation is shown by the size exclusion chromatograms of the reaction products formed between excess IL emim-DCN and Phenyl Glycidyl Ether (PGE) in Figure 1 1 which shows the disappearance of the peak associated with PGE and the appearance of a higher molecular weight species after reaction. The reaction between ionic liquid and epoxy can be monitored isothermally using DSC. Figure 10 shows the isothermal conversion profiles for 100 °C and 120 °C which appear to follow second order kinetics.

As the concentration of epoxy in the initial reaction mixture is increased the viscosity and molecular weight of the product increases and the concentration of unreacted IL decreases. For difunctional epoxies it was observed that as a 1 : 1 molar ratio of epoxy moiety to IL is approached, a room temperature elastomer is formed (see example). Such materials are potentially crosslinked polymers that may contain unreacted IL as well as low molecular weight reaction products including the adducts described in the previous paragraph. As the concentration of IL is further decreased, cross-linked polymers with glass transition temperature (Tg) above room temperature are produced with little or no remaining unreacted IL. Lower concentrations of IL result in higher Tg and the fracture toughness increases with increasing IL content (see examples)

The properties of the resulting polymers are also dependent on the functionality and type of epoxy. Higher molecular weight epoxies of the same class (DGEBA) produce lower Tg products when cured using the same molar ratio of IL to epoxy moiety. At the same time the higher molecular weight epoxies produce higher toughness polymers. Additionally, monofunctional epoxies result in systems that are flowable and potentially linear and/or branched while epoxies with epoxy functionality greater than 2 will form crosslinked products at lower weight fractions of ionic liquid.

The characteristic double exotherm exhibited by the DSC ramp experiments (multiple figures) suggests a cure process comprised of at least two reaction steps. The formation of an adduct is likely the first step as we have demonstrated its existence by SEC and by the preparation of stable products by the reaction of 1 : 1 molar ratio mixtures of epoxy to ionic liquid. It has further been found that the addition of such adduct to epoxy results in polymerization. This is shown by the reaction exotherm measured by DSC of a mixture comprised by 50 wt% Epon™ 828 DGEBA and 50 wt% previously reacted adduct of 1 : 1 molar ratio of PGE:emimdcn of Figure 7.

The ionic liquid may serve as a curing agent for the epoxy resin, as a curing accelerator when combined with any other curing agent or as a modi fier for the epoxy resin. Accordingly, it is desirable that the amount of the ionic liquid is suitably controlled, particularly when the ionic liquid is used for tuning the properties of the cured products. A more preferred range of use of the ionic liquid is at a molar ratio of from 0.1 to 10 moles of ionic liquid per 2 moles of epoxy groups, even more preferably a molar ratio of from 0.2 to 5, still more preferably from 0.2-2.0 is employed.

Thus, the present invention contemplates embodiments where ionic liquids are used to cure epoxy resins. Alternative embodiments form mixtures of ionic liquids and epoxy resins in order to prepare adducts of the ionic liquid and epoxy resin and then the mixtures containing the adducts are subsequently used to cure epoxy resins. It is also within the scope of the present invention to include a stoichiometric excess of ionic liquid relative to epoxy resin to ensure the presence of one or both of unreacted ionic liquid or adducts of ionic liquid and epoxy resin in the resultant products. Alternatively, non- reactive ionic liquids which do not cure the epoxy resin may be included in the compositions of the present invention in addition to the reactive ionic liquids used to cure the epoxy resin to ensure the presence of unreacted non-reactive ionic liquid in the cured product. These variations can be used to adjust the properties of the resultant cured product such as the fracture toughness, elasticity, and conductivity.

Amounts of 0.01 up to about 20 wt% of a RT1L mixed with epoxy resin exhibits good resin miscibility, long pot life and high thermal stability while being able to initiate cure at elevated temperatures without the associated problems of dispersion and nonhomogenous cure encountered with initiators that are solid at room temperature.

The ionic liquid may serve as a curing agent for the epoxy resin, as a curing accelerator when combined with any other conventional epoxy curing agent, such as para- amino cyclohexyl methane (PACM) or as a modifier for the epoxy resin. Accordingly, it is desirable that the amount of the ionic liquid is suitably controlled, particularly when the ionic liquid is used for tuning the properties of the cured products.

Some ionic liquids are referred to as "hydrophobic" ionic liquids. Hydrophobic ionic liquids are those that form biphasic mixtures in combination with water. However, the miscibility of ionic liquids and water is affected by temperature and thus a biphasic mixture can potentially become completely miscible in water at an elevated temperature. Hydrophilic ionic liquids are those that are completely miscible with water at or below room temperature, i.e. 23°C. All ionic liquids are hygroscopic and thus absorb water from the environment.

The polymers are prepared by mixing suitable ionic liquids with epoxy resins and heating to appropriate temperatures. The ionic liquids can be reactive (i.e. capable of initiating polymerization) and/or non-reactive. If only non-reactive ionic liquids are used then a separate curing agent must be used. Any of a wide variety of epoxy resins may be employed in the present invention. Epoxy resins as referenced herein are resins which include one or more glycidyl ether groups, including linear, branched or cyclic epoxies. The epoxy resins may include one glycidyl ether group in which case a linear polymer may be formed. The epoxy resin may alternatively include two, three or more glycidyl ether groups in which case cross-linked materials can be formed. Mixtures of epoxy resins containing different numbers of glycidyl ether groups may also be employed in the present invention. The glycidyl ether groups allow curing of the epoxy resins to increase the molecular weight of the cured product. In certain embodiments, the epoxy resins are cross-linked using the glycidyl ether groups as reactive cross-linking sites. Additionally non-crosslinked systems can be prepared by using monofunctional epoxy monomers. One example of such a monofunctional monomer is phenyl glycidyl ether (PGE). Such monofunctional epoxies can also be used in combination with polyfunctional epoxy monomers like Novolacs and N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM).

Exemplary suitable epoxy resins include, but are not limited to. glycidyl ethers of polyphenols such as bisphenol A, bisphenol F, bisphenol AD, catechol, resorcinol;

polyglycidyl ethers prepared by reacting a polyalcohol such as glycerin or polyethylene glycol, and epichlorohydrin; glycidyl ether esters prepared by reacting a hydroxycarboxylic acid such as p-hydroxybenzoic acid or β-hydroxynaphthoic acid, and epichlorohydrin;

polyglycidyl esters prepared by reacting a polycarboxylic acid such as phthalic acid or terephthalic acid, and epichlorohydrin; and further epoxidated phenol-novolak resins, epoxidated cresol-novolak resins, epoxidated polyolefins, cycloaliphatic epoxy resins and other urethane-modified epoxy resins, to which, however, the invention should not be limited.

Commercial epoxy resin products are, for example, Miller-Stephenson's Epon™ 828, Epon™ 836 and Epon™ 100 I F, Japan Epoxy Resin's Epikote 828, 1001 , 801 , 806. 807. 1 52, 604, 630, 871 , YX8000, YX8034, YX4000, Cardula El OP; Dai-Nippon Ink Industry's Epiclon 830, 835LV, HP4032D, 703, 720, 726, HP820; Asahi Denka Kogyo's EP4100, EP4000, EP4080, EP4085, EP4088, EPU6, EPR4023, EPR 1309, EP49-20; and Nagase ChemteX's Denacol EX41 1 , EX314, EX201 , EX212, EX252, EX 1 1 1 , EX 146, EX721 , to which, however, the invention should not be limited. A particularly preferred epoxy resin useful in the present invention is the diglycidyl ether of bisphenol A (DGEBA). One or more of these may be used either singly or as combined.

The above-mentioned epoxy resins may have any other functional group than the epoxy group. For example, the epoxy resins may additionally include a hydroxyl group, a vinyl group, an acetal group, an ester group, a carbonyl group, an amide group, an alkoxysilyl group or two or more of such groups including mixtures thereof.

Preferably amounts of 0.01 up to about 20 wt % of a RTIL mixed with epoxy resin exhibits good resin miscibility, long pot life and high thermal stability while being able to initiate cure at elevated temperatures without the associated problems of dispersion and nonhomogenous cure encountered with initiators that are solid at room temperature. Any suitable method for curing may be employed. As a practical matter, to avoid product deterioration and overuse of energy, curing is typically conducted at a temperature of from about 50°C to about 250°C, more preferably, from about 55° C to about 200° C, even more preferably, from about 80° C to about 140° C. Gelling time is preferably up to 1 20 minutes, more preferably up to 90 minutes, even more preferably up to 60 minutes, still more preferably up to 30 minutes and may be as short as up to 15 minutes. The lowermost limit of the gelling time may be as short as 0.001 seconds, more preferably 0. 1 seconds. For relatively instant curing, the upper limit of the gelling time is preferably 1 5 minutes, more preferably 5 minutes, even more preferably 3 minutes, still more preferably 1 minute, further more preferably 30 seconds.

The composition of the invention is expected to be stored at a temperature of from 20° C to 40° C. Compositions comprising the resin and the ionic liquid may be stored at these temperatures from 3 hours to 6 months, more preferably, from 3 days to 3 months.

Various additives may be added to the resin composition of the invention, not detracting from the advantages thereof, and they include fillers, diluents, solvents, pigments, flexibility-imparting agents, coupling agents, antioxidants, defoaming agents, etc. Such additives may be employed in conventional amounts and may be added directly to the process during formation of the cured composition. Illustrative of such additives are diluents, solvents, coupling agents, defoaming agents, colorants, pigments, carbon black, fibers such as glass fibers, carbon fibers and aramid fibers, fillers, impact modifiers, antioxidants, stabilizers, flame retardants, reheating aids, crystallization aids, oxygen scavengers, plasticizers, flexibilizers, nucleating agents, foaming agents, mold release agents, and combinations thereof. The cured resins may be blended with other polymers or foamed by any conventions foaming means, if desired.

Tunable properties of the composites manufactured by methods in accordance with the present invention include the ability to vary cross-linker ratios, the ability to vary sulfonic acid concentration when using non-reactive RTILs with this functionality, and RTI L concentration. The result of this is the ability to produce composites with tunable thermal, mechanical and conductive properties.

Thermomechanical analyses show that materials with glass transition temperatures

(Tg) of ~200°C (tan δ max) can be obtained for DGEBA systems and that the Tg and cross- linking density are dependent on the concentration of the RTIL used. Higher values are possible using higher functionality epoxy monomers. These results have been confirmed by differential scanning calorimetry over a range of RTIL concentrations. Gravimetric analysis also indicates that the hydrophilicity of the cured networks is dependent on the concentration of RTIL used. This demonstrates that a number of important resin properties can be customized by adjustment of the concentration of the RTIL in the process.

The type and/or amount of ionic liquids of the present invention can be selected to allow control of various physicochemical properties of the polymers such as glass transition temperature, cross-linking density, electrical conductivity, thermal stability, specific gravity and the heat capacity. Selection of the type and amount of ionic liquid can also be used to tune the vapor pressure, curing temperature, curing time, solvating characteristics, adduct formation, heat of reaction and hydrophilicity during the curing reaction.

The choice of cation and anion for the ionic liquid may be used alone, or in combination with a selection of a specific amount of a particular ionic liquid, to determine physical properties such as melting point, viscosity, density and water solubility. Melting point can be easily modified, as shown in the examples, by structural variation of one of the ions in the ionic liquid or by combining different ions to form the ionic liquid. For example, as shown in Figure 12, the resin viscosity of Epon™ 828 can be varied as a function of ionic liquid concentration.

The process of the present invention can be employed to prepare epoxy-containing polymers with high fracture toughness. Moreover the reaction is latent meaning that the mixture of ionic liquid and epoxy resin remains stable at room temperature for prolonged periods of time reacting rapidly once the temperature is raised to a threshold level. The advantages include: ( 1 ) The ability of the latent initiator to fully dissolve in epoxy resins, (2) The long term stability of the mixtures of I L and epoxy resins, and (3) excellent mechanical properties of resulting polymers, particularly fracture toughness.

These epoxy-containing materials could be advantageous for use in adhesives, coatings, as encapsulants for electronic materials, and as matrix materials for fiber reinforced composites prepared by filament winding, processes for preparing pre-pregs, and liquid molding processes.

The invention will now be further illustrated by the following non-limiting examples.

EXAM PLES

Example 1 - Curing of Epoxy Resins with RTILs

Following room temperature ionic liquids in accordance with the present invention were demonstrated to initiate epoxy polymerization and produce mechanically stable polymers. Resin Preparation

Epon™ 828 and RTILs were treated using 12A molecular sieves for 24 hours at room temperature and left in the oven at 1 10°C overnight to remove water and then used. Epon™ 836 and Epon™ 100 I F were used as purchased.

Resin Cure

The resin samples were cured in stainless steel mold with approximate cavity dimensions of 15 mm x 50 mm x 5 mm for DMA testing. Samples were also prepared in stainless steel molds with approximate dimensions of 90 mm x 150 mm x 5 mm and 90 mm x 150 mm x 7 mm respectively, for testing flexural and fracture toughness. A mold release agent was applied to the molds before use. The cure schedule was as follows. After placing the filled mold in an oven at 80°C the temperature was 'increased from 80°C to 1 20°C over 30 minutes then kept at 120°C for 3 hours, during this time the resin turns from clear to dark brown. Following the 3 hour period at 120°C the oven was heated to 1 50°C over 10 minutes and kept at this temperature for 2 hours. The cured samples were dark brown in color. NI R spectroscopy shows the complete disappearance of the epoxy group.

1. Characterization

A. Differential Scanning Calorimeter Analysis (DSC)

The heat of reaction and number of stages of the Epon™ 828/RTIL reaction were measured by a TA instrument Model Q2000. The instrument was equipped with a 50ml/min nitrogen gas purge flow. An approximately 6- 10 mg sample of the curable resin/ionic liquid mixture was placed inside the Tzero Aluminum Hermetic pan and sealed with the Tzero hermetic lid by a Tzero press. An empty Tzero hermetic pan with sealed lid was used as a reference. The standard test mode was used and the samples were heated at a heating rate of 2°C/min from 30°C to 250°C. Heat flow of the reaction was calculated by integration of the exothermic peaks with respect to the baseline and the number of stages of each reaction was determined by the number of visible exothermic peaks during the first temperature ramp.

B. Dynamic Mechanical Analysis (DMA)

The thermo-mechanical properties of these Epon™ (828, 836 and 1001 F)/ionic liquid cured samples were measured by using dynamic mechanical analysis. Rectangular samples were prepared having approximate dimensions of 20 mm x 10 mm x 3 mm. These samples were tested using a TA instrument Q800 model in a single cantilever clamp, multi-frequency- strain mode, at 1 Hz with a deflection of 15 μιη. A heating rate of 2°C/min from 30°C to 250°C was applied. Two temperature ramp experiments were run for each sample. The first run was performed to post cure the sample. The temperature peak of the second run at the loss modulus was considered to be the glass transition temperature (T_g) and the storage modulus of (T_g+50°C) was taken as the rubbery modulus of the polymer material. C. Polymer Mechanical Testing

The flexural tests were based on the ASTM D790-03, Test Method I and the Procedure A standard to determine the flexural modulus and flexural strength of the material. Rectangular samples were prepared with the dimensions of about 6.25 cm x 1 .25 cm x 0.32 cm and were tested on the flat side, face up with a 5.1 cm support span and a support-to-depth ratio of 16. The samples were tested at room temperature, approximately 22-25°C using an Instron 8872 instrument at a crosshead speed of 1.35 mm/min.

The fracture toughness tests were based on the ASTM 5045-99 method. Rectangular samples were prepared with the dimensions of 6.25 cm x 1 .25 cm x 0.64 cm. The notch was placed at the halfway point of the sample length and width using a table saw with a diamond blade for 5 test samples; one un-notched sample was tested as a control. A sharp razor blade was used to initiate a crack at the surface of the notch just before the testing. Samples were tested at room temperature, approximately 22-25°C using an Instron 8872 instrument at a crosshead speed of 10 mm/min with a span length of about 5.1 cm. The sample dimensions were measured after the testing to measure the notch length accurately.

Tabic 1 - DMA results of E ons™ 828, 862, 1001 F cured with Ionic Resins

Table 2 - Mechanical Properties of emimden and Epons™ (836, 1001 F) cured e ox resins

Various thermomechanical characteristics and mechanical properties of the epoxy-containing resins were measured and are given in Tables 1 -2. These characteristics and properties show that these epoxy-containing resins are suitable for a wide variety of different applications.

Table 2 shows, for example, that, as measured in accordance with ASTM D 790, the ionic liquid content of the resultant epoxy-containing polymer directly affects the flexural strength of the epoxy-containing polymer. Cationically cured epoxy resins typically exhibit a flexural strength of about 70 Mpa. Table 2 shows that flexural strengths significantly higher than 70 Mpa can be achieved using the curing method of the present invention.

With regard to the fracture toughness, as measured in accordance with ASTM D 5045-99, it has been shown that fracture toughness increases significantly with ionic liquid content in the epoxy-containing polymer. Epoxy-containing polymers having very high fracture toughness were obtained using the method of the present invention, as shown in Table 2.

1

Example 2 - Step-growth Polymerizations in Non-Rcactivc Ionic Liquids

Ionic liquids that dissolve in, but do not react with, epoxide groups may be used to disperse ionic groups within the network structure to modify properties. An example of this is EPON™ 828-PACM cured in the presence of non-reactive but soluble ionic liquids.

Two examples of such ionic liquids are (emim)ethylsulfate and (emim)tosylate. Figure 6 is a plot of the Tg and storage modulus of fully cured epoxy-am ine thermosets in the presence of (emim)ethylsulfate and (emim)tosylate compared to epoxy-amine

thermosets cured without any ionic liquid. For thermosets cured with either of these ionic liquids, there is a distinct increase in the storage modulus at room temperature

(glassy modulus) suggesting possible interna! anti-plasticization effects. The loss modulus peaks indicate a significant depression in the Tg for both (emim)ethylsulfate and

(emim)tosylate. Similar behavior has been observed for PMMA in the presence of ionic

liquids and is considered to be due to increased chain mobility in the vicinity of low molecular weight solvents which lower the overall Tg.

Example 3

In Example 3, a model epoxy resin, polyglycidyl ether (PGE), was cured in the presence of emimdcn to study the effects of the presence of excess ionic liquid during the curing reaction.

The heat of reaction for excess ionic liquid relative to epoxy compound is plotted in Figure 9.

The isothermal conversion profiles obtained by differential scanning calorimetry for a reaction of 1 .4 mol of emimdcn per mole of PGE epoxy resin are shown in Figure 10.

Figure 1 1 shows the size exclusion chromatograph of the PGE epoxy resin cured with excess emimdcn demonstrating adduct formation.

These figures demonstrate that various aspects of the curing reaction can be controlled by using a controlled excess of ionic liquid relative to the epoxy resin during the curing reaction. Example 4

In this example, the viscosity of Epon™ 828 was varied as a function of ionic liquid concentration to show that the viscosity is a tunable property. Figure 1 2 shows a graph of the viscosity, measured using concentric cylinder geometry, at a I ^s"1 shear rate at 25°C. As can be seen, increasing the ionic liquid concentration resulted in a decrease in viscosity.

Example 5

In this example, the heat flow during the curing reaction was monitored as a function of the particular cation employed in the ionic liquid. The results are shown in Figure 1 3. As can be seen, the curing reaction can be tuned by select ion of an appropriate cation for the ionic liquid. Examples 6-7

Sample Preparation

A 40 wt% mixture of IL/epoxy resin was may by mixing 1 .1966 grams of emimden IL with 1 .7945 grams of Epon™ 828.

A 45 wt% mixture of IL/epoxy resin was may by mixing 1 .3874 grams of emimden I L was mixed with 1 .6534 grams of Epon™ 828.

The materials were mixed for 2 minutes 2000 rpm, capped and at room temperature poured into open mold. The molds were placed in oven at 90°C for 45 minutes and the temperature was slowly ramped at 10°C/20 minutes until 1 20°C was reached. The oven was held at 120°C for 2 hours and the samples were removed from the oven and cooled to room temperature. Finally, the samples were removed from the mold.

Results

Both the 45wt% and 40 wt% I L/Epon™ 8 28 samples yielded rubbery plaques of nominal dimensions of (diameter=36mm, thickness 1 .9mm). When bent and released, the samples returned to their original shape as shown in Figures 14A- 15 B. No signs of plastic deformation were observed. 1

The foregoing examples have been presented for the purpose of illustration and description only and are not to be construed as limiting the invention in any way. The scope of the invention is to be determined from the claims appended hereto.

Claims

CLAIMS;

1. A resin composition comprising:

(a) at least one epoxy resin; and

(b) at least one ionic liquid, an adduct of an ionic liquid and an epoxy resin, or a mixture thereof, wherein when component (b) comprises an ionic liquid, the ionic liquid comprises at least one cation selected from the group consisting of an imidazolium ion of the formula (I):

wherein Ri is selected from the group consisting of methyl, C₃-C1₀ linear or branched alkyl groups, cycloalkyl groups having up to 10 carbon atoms, and C1-C1₀ alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups, and R2 is selected from the group consisting of C1-C10 linear or branched alkyl groups, cycloalkyl groups having up to 10 carbon atoms, and C1-C1₀ alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups;

a piperidinium ion; a pyrrolidinium ion; a pyrazolium ion; a guanidinium ion and a pyridinium ion and at least one anion selected from the group consisting of a dicyanamide anion, an ethyl sulfate anion and a bis(trifluoromethylsulfonyl) imide anion.

2. A resin composition as claimed in claim 1, wherein when component (b) comprises an ionic liquid, the cation is selected from the group consisting of an imidazolium ion of the formula (I):

R₂

wherein Ri is selected from the group consisting of methyl, C3-C10 linear or branched alkyl groups, cycloalkyl groups having up to 10 carbon atoms, and C1-C10 alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups, and R2 is selected from the group consisting of C1-C1₀ linear or branched alkyl groups, cycloalkyl groups having up to 10 carbon atoms, and C1-C10 alkyl groups containing at least one substituent selected from hydroxyl, cyano, vinyl, acrylate, methacrylate, epoxy, ether and carboxyl groups.

3. A resin composition as claimed in claim 2, wherein the anion is a dicyanamide anion.

4. A resin composition as claimed in claim 1 , wherein the epoxy resin comprises monoepoxy resin.

5. A resin composition as claimed in claim 1, wherein the epoxy resin comprises a diepoxy resin.

6. A resin composition as claimed in claim 1, wherein the molar ratio of ionic liquid to epoxy resin is from 0.1 to 10.

7. A resin composition as claimed in claim 1, wherein the molar ratio of ionic liquid to epoxy resin is from 0.1 to 1.0.

8. A resin composition as claimed in claim 1, wherein the molar ratio of ionic liquid to epoxy resin is from 1 to 10.

9. A resin composition as claimed in claim 1, wherein component (b) comprises an ionic liquid.

10. A resin composition as claimed in claim 1, wherein component (b) comprises an adduct of an ionic liquid and an epoxy resin.

11. A resin composition as claimed in claim 1, wherein component (b) comprises a mixture of an ionic liquid and an adduct of an ionic liquid and an epoxy resin.

12. A cured product, which is obtained by curing a resin composition as claimed in claim 1.

13. A cured product, which is obtained by curing a resin composition as claimed in claim 2.

14. A cured product, which is obtained by curing a resin composition as claimed in claim 3.

15. A resin composition as claimed in claim 1, wherein the ionic liquid comprises at least one salt selected from the group consisting of:

1 -butyl-3 -methylimidazolium dicyanamide;

1 -hexyl-3 -methyl- imidazolium dicyanamide;

1 -(2 -hydroxyethyl)-3 -methylimidazolium dicyanamide;

1 -(3 -cycnopropyl)-3 -methylimidazolium dicyanamide; and

1 -ethyl-3 -methylpyrrolidinium dicycanamide.

16. A resin composition comprising:

(a) at least one epoxy resin; and

(b) at least one ionic liquid, an adduct of an ionic liquid and an epoxy resin, or a mixture thereof, wherein the ionic liquid comprises at least one cation selected from the group consisting of an imidazolium ion; a piperidinium ion; a pyrrolidinium ion; a pyrazolium ion; a guanidinium ion and a pyridinium ion and a dicyanamide anion, with the proviso that when component (b) comprises an ionic liquid and the ionic liquid is 1 -ethyl-3 -methylimidazolium dicyanamide the epoxy resin is not Epon™ 828.

17. A resin composition as claimed in claim 16, wherein the epoxy resin comprises monoepoxy resin.

18. A resin composition as claimed in claim 16, wherein the epoxy resin comprises a diepoxy resin.

19. A resin composition as claimed in claim 16, wherein the molar ratio of ionic liquid to epoxy resin is from 0.1 to 10.

20. A resin composition as claimed in claim 16, wherein the molar ratio of ionic liquid to epoxy resin is from 0.1 to 1.0.

21. A resin composition as claimed in claim 16, wherein the molar ratio of ionic liquid to epoxy resin is from 1 to 10.

22. A resin composition as claimed in claim 16, wherein component (b) comprises an ionic liquid.

23. A resin composition as claimed in claim 16, wherein component (b) comprises an adduct of an ionic liquid and an epoxy resin.

24. A resin composition as claimed in claim 16, wherein component (b) comprises a mixture of an ionic liquid and an adduct of an ionic liquid and an epoxy resin.

25. A cured product, which is obtained by curing a resin composition as claimed in claim 16.

26. A resin composition comprising:

(a) at least one epoxy resin; and

(b) at least one ionic liquid, an adduct of an ionic liquid and an epoxy resin, or a mixture thereof, wherein when component (b) comprises an ionic liquid, the ionic liquid comprises 1 -alkyl-3 -methylimidazolium chloride.

27. A resin composition as claimed in claim 26, wherein the ionic liquid comprises 1 -hexyl-3 -methylimidazolium chloride.